CHEMICAL-POTENTIAL PARADOX IN MOLECULAR SIMULATION - EXPLANATION AND MONTE-CARLO RESULTS FOR A LENNARD-JONES FLUID

Monte Carlo simulation results are presented for uniform fluids which show that the discrepancies in the values of the chemical potential fo und by the Widom and the Shing-Gubbins methods arise from the introduc tion of molecular cores to prevent too close approach of the molecules . For dense fluids of molecules interacting according to a modified Le nnard-Jones potential, excellent agreement(to <0.1kT) is found at redu ced temperature (T) over tilde = kT/epsilon = 1 between the directly c omputed discrepancies and the results of a treatment based on 'overlap ping' of cores provided that the core diameter is greater than or equa l to 0.875 sigma where sigma is the Lennard-Jones collision diameter. In the same upper range of core sizes the dependence of the discrepanc y on density also agrees well with the overlapping theory at least dow n to a relative density (<(rho)over tilde> = rho sigma(3)) of 0.4. For small core sizes the discrepancy is, to a very good approximation, in dependent of core diameter, and this is explained in terms of 'poor st atistics'. Results for (T) over tilde = 2.5 are generally similar to t hose for (T) over tilde = 1, but agreement with the overlapping theory is not as good.